1. Field of the Invention
The present invention relates to a magnetic memory cell and a method for accessing the same, and more specifically to a magnetic memory cell used for constructing a voluntarily writable/readable nonvolatile random access memory, and a method for accessing the memory.
2. Description of Related Art
In the prior art, a nondestructive memory composed of a pair of magnetic thin films was proposed. In brief, two magnetic films having different coercive forces are stacked with a non-magnetic insulating film interposed between the two sheets of magnetic films. The film having a large coercive force is used for holding one item of information, and the film having a small coercive force is used only for a reading of the stored information. The two magnetic films are magnetostatically coupled to each other, so that a magnetization of the read only magnetic film is controlled by a magnetic field produced by the information holding magnetic film.
Referring to FIGS. 1A and 1B, two different conditions of a magnetic coupling between the information holding magnetic film and the read only magnetic film are diagrammatically illustrated. The two conditions of a magnetic coupling are distinguished from each other by a direction of magnetization. FIG. 1A shows a "0" storage condition, and FIG. 1B shows a "1" storage condition. The non-magnetic insulating film is omitted in the figure.
It is assumed that a reading magnetic field is applied in a direction from left to right in the figure. In this case, a reading voltage from the "0" storage condition is "0". On the other hand, when the "1" storage condition is read out, an output voltage due to a magnetization reversal of the read only film is detected. If the reading magnetic field disappears, the magnetization of the read only magnetic film is returned to an original condition, and the stored information is held. In the case of writing information, a magnetic field larger than the coercive force of the information holding magnetic film is applied so as to change the condition of the memory cell.
The above mentioned memory cell is of a nonvolatile and nondestructive type. With microminiaturization of a cell size for increasing a recording density and an integration density, a detection sensibility of a magnetic stored information is inevitably lowered, and an electric power consumption due to a control current is increased. As a result, the increase of the reading density has been limited on the order of 1Mbits/cm.sup.2. A mechanism of magnetization rotation in the magnetic material is based on displacement of a magnetic domain wall. Therefore, an access speed is on the order of microseconds to milliseconds, and is less than one-hundredth of that of a semiconductor memory.